Synthetic resin worm or screw presses of the aforedescribed type are known and include the systems described in the following U.S. Pat. Nos. 3,647,329 and 3,910,316.
Such synthetic resin worm presses, as noted, comprise a generally cylindrical housing and a worm rotatable in this housing and comprising a driven worm shaft formed with or constituting the worm core from which at least one worm or screw thread, also called a rib or flight, rises.
The thread height and the pitch angle of the thread varies along the length of the worm so that different degrees of compression and shearing action may be applied to the material and thus different zones of the worm subject the material to different processing parameters.
In general, a filling opening communicates laterally with the worm cylinder through a wall of the housing and this zone can be referred to as a feeding zone. Adjacent the feeding zone in the direction of displacement of the worm is an intake zone in which the synthetic resin material which passes into the worm cylinder in the intake zone is entrained by the flight or thread and forced to move along the length of the worm.
In general, the thread or flight in the intake zone and in the feed opening zone have the same pitch angle. The pitch angle in the intake zone, however, can be larger.
In conventional synthetic worm presses (see Schenkel: Kunststoff-Extrudertechnik, 1963, page 171, issue 5 /72), the thread or flight not only in the feed opening zone but also in the region of the intake zone has an increased thread height (or depth) as measured from the crest of the thread along a radius of the worm to the root, i.e. the thread height T is equal to one half the difference between the diameter of the worm D less the root diameter d of the worm. The root diameter of the worm, i.e. the diameter of the core is also the same in the feed opening zone and the intake zone and a small transition region is provided between the interior of the cylinder at the intake zone and its junction with the feed opening zone. Up to now these have been the considerations that the art found necessary for a high rate of flow of the material into and through the intake zone (see Schenkel, pages 104-113).
Analytic investigations have also suggested that a high rate of displacement through the intake zone for a given rotary speed of the worm, in particular, depends upon a high thread height, a low coefficient of fraction of the worm surface, a high coefficient of friction of the inner wall of the cylinder and accommodation of the pitch angle to the effective value of the friction coefficient. Grooves have been avoided in the intake zone heretofore because they were believed to reduce displacement efficiency, especially with greater depths of such grooves.
To increase the flow through the intake zone, it is known in practice to provide the feed opening with additional pockets which are more or less of crescent shape and which are formed in the housing.
While they have been found to be helpful in some cases, by and large these earlier proposals have not been satisfactory, especially when there is a feedback of pieces of the synthetic resin to the worm.